Speed Kills

The engine in car is spinning at 2350 RPMs as it travels down the highway in 4th gear. Its speed is 55 mi/hr. The differential has a 3.73 gear ratio meaning that the drive shaft spins 3.73 times for every 1 turn of the wheels.

The enthusiastic owner wants to soup it up a bit and changes the rear to a 4.11 gear ratio or 4.11 turns of the driveshaft to one spin of the wheels. He also replaces the rear tires with slicks that are 2’’ more in diameter (outside edge to outside edge=28"). If the car travels in 4th gear at the same RPMs down the highway, how much faster/slower will the car be traveling?

The increase in tire size will increase the circumference of the wheel by 1.0769231x, thus increasing the speed of the car to 59.23077 mph.
The change in the gear ratio will reduce the wheel rotation to .907543x, reducing the speed from 59.23077 to 53.75445 mph, for an overall reduction in speed.